Flag=1
num=0
while Flag:
    if(num==2):
        Flag=0
    id=input("请输入账号")


    if(id=='a'):
        num = 0
        password=input('请输入密码')
        if(password=='111'):
            print('欢迎')
        else:
            print("密码错误")
            continue
    elif (id == 'b'):
        num = 0
        password = input('请输入密码')
        if (password == '222'):
            print('欢迎')
        else:
            print("密码错误")
            continue
    elif (id == 'c'):
        num = 0
        password = input('请输入密码')
        if (password == '333'):
            print('欢迎')
        else:
            print("密码错误")
            continue
    else:
        print("用户不存在！")
        num+=1
        continue
print('程序结束')